From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: shm_toc_lookup API
Date: Mon, 05 Jun 2017 14:57:10 -0400
Message-ID: <28314.1496689030@sss.pgh.pa.us> (raw)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Jun 5, 2017 at 12:19 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> In practice it probably can't fail even if 64-bit reads aren't atomic,
>> simply because we'll never have enough entries in a shm_toc to make the
>> high-order half ever change. But that just begs the question why the
>> field is declared Size rather than int. I think we should make it the
>> latter.
> Yeah. I think a shm_toc with more than 2^10 entries would probably
> perform badly enough that somebody would rewrite this entire module,
> so we don't really need to worry about having more than 2^31.
> Changing to int (or uint32) seems fine.
Done with uint32.
>> I am also thinking that most of the shm_toc functions need to have the
>> toc pointers declared as "volatile *", but particularly shm_toc_lookup.
(actually, they do already use volatile pointers, except for shm_toc_lookup)
>> That read_barrier call might prevent the hardware from reordering
>> accesses, but I don't think it stops the compiler from doing so.
> If it doesn't prevent both the hardware and the compiler from
> reordering, it's broken. See the comments for pg_read_barrier() in
> atomics.h.
Meh. Without volatile, I think that the compiler would be within its
rights to elide the nentry local variable and re-fetch toc->toc_nentry
each time through the loop. It'd be unlikely to do so, granted, but
I'm not convinced that pg_read_barrier() would prevent that.
However, as long as the write barrier in shm_toc_insert does what it's
supposed to, I think we'd be safe even if that happened. So probably
it's a moot point.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com
Subject: Re: shm_toc_lookup API
In-Reply-To: <28314.1496689030@sss.pgh.pa.us>
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